null input parameters custom function

Hello dufoq3 

thanks for your recommendation, I will use it.

However, I still have problems with this code.

I am writing down the final code I have and how I am invoking it:

(
Input_Table as table,
Adder_Table as table,

Input_Table_Column1 as text,
Input_Table_Column2 as text,
Input_Table_Column3 as text,
Adder_Table_Column1 as text,
Adder_Table_Column2 as text,
Adder_Table_Column3 as text,
Adder as text,

optional Input_Table_Column4 as text,
optional Input_Table_Column5 as text,
optional Adder_Table_Column4 as text,
optional Adder_Table_Column5 as text
) =>

let
Input_Table_Helper_Column = Table.AddColumn(Input_Table, "HelperLPInput", each
Text.Combine(List.Select({
Record.FieldOrDefault(_, Input_Table_Column1,""),
Record.FieldOrDefault(_, Input_Table_Column2,""),
Record.FieldOrDefault(_, Input_Table_Column3,""),
Record.FieldOrDefault(_, Input_Table_Column4,""),
Record.FieldOrDefault(_, Input_Table_Column5,"")
}, each _ <> null), " "), type text),

Adder_Table_Helper_Column = Table.AddColumn(Adder_Table, "HelperLPAdder", each
Text.Combine(List.Select({
Record.FieldOrDefault(_, Adder_Table_Column1,""),
Record.FieldOrDefault(_, Adder_Table_Column2,""),
Record.FieldOrDefault(_, Adder_Table_Column3,""),
Record.FieldOrDefault(_, Adder_Table_Column4,""),
Record.FieldOrDefault(_, Adder_Table_Column5,"")
}, each _ <> null), " "), type text),

// Merge and Expand
Merged_Table = Table.NestedJoin(
Input_Table_Helper_Column,
{"HelperLPInput"},
Adder_Table_Helper_Column,
{"HelperLPAdder"},
"Adder",
JoinKind.LeftOuter
),

Input_Table_Headers = Table.ColumnNames(Input_Table),
Adder_Table_Headers = Table.ColumnNames(Adder_Table),
List_Header_Difference = List.Difference(Adder_Table_Headers, Input_Table_Headers),

Expanded_Merged_Table = Table.ExpandTableColumn(
Merged_Table,
Adder,
List_Header_Difference,
List_Header_Difference
),

Remove_HelperLPInput = Table.RemoveColumns(Expanded_Merged_Table, "HelperLPInput")

in
Remove_HelperLPInput

Invoking

let
Source = MergeLP_3_Test(Total_Sales_DOLLARS_TEST, PCBA_Type_List_Price_Adder, "Region", "Project_Step", "Fail_Safe_Description", "Region", "Project_Step", "Fail_Safe_Description", "Adder")
in
Source

Thanks for your help.